The present invention relates generally to medical diagnostic imaging systems, and in particular relates to a system and method for the detection and compensation of scintillator hysteresis caused by electrical charge stored in charge trapping centers of scintillator material.
X-ray imaging has long been an accepted medical diagnostic tool. X-ray imaging systems are commonly used to capture, as examples, thoracic, cervical, spinal, cranial and abdominal images that often include information necessary for a physician to make an accurate diagnosis. X-ray imaging systems typically include an x-ray source and an x-ray detector. When having a thoracic x-ray image taken, for example, a patient stands with his or her chest against the x-ray sensor as an x-ray technologist positions the x-ray detector and the x-ray source at an appropriate height. X-rays produced by the source travel through the patient's chest, and the x-ray detector then detects the x-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system obtains the detected x-ray energy from the x-ray detector and prepares a corresponding diagnostic image on a display.
In addition, x-ray images may be used for many other purposes. For instance, internal defects in a target object may be detected. Additionally, changes in internal structure or alignment may be determined. Furthermore, the image may show the presence or absence of objects in the target. The information gained from x-ray imaging has applications in many fields other than medicine, including manufacturing.
The x-ray detector may be a conventional screen/film configuration, in which the screen converts the x-rays to light that exposes the film. The x-ray detector may also be a solid-state digital image detector. Digital detectors afford a significantly greater dynamic range than conventional screen/film configurations.
An exemplary digital x-ray detector is shown in FIG. 1. Digital detectors 100 generally include a substrate material 110, a matrix of picture elements 120, a scintillator 130, readout electronics 140 and electrical connects 150. The matrix of picture elements (pixels) 120 is placed between the substrate material 110 and the scintillator 130. The matrix of pixels 120 is electronically connected to the readout electronics 140 by the electrical connects 150. The matrix of pixels 120 includes multiple pixels. Each pixel typically includes a photodiode 125.
The scintillator 130 is composed of x-ray converter material that converts incident x-ray flux into light. An x-ray flux source directs x-ray flux to the detector 100. The x-ray flux then strikes the detector 100. When the x-ray flux strikes the detector 100, the scintillator 130 converts the x-ray flux into light. The light then passes down to the matrix of pixels 120. Each photodiode 125 is initially charged with a known amount of reverse bias voltage. Once the light strikes the photodiodes 125 contained in the matrix of pixels 120, the light discharges some or all of the stored reverse bias voltage. Photodiodes 125 that receive greater intensity of incident light discharge a greater amount of initially stored voltage than photodiodes 125 that receive a lesser intensity of incident light. In an x-ray imaging of a human bone, for example, denser areas of the human bone block more x-rays from striking the detector 100 than areas of the detector 100 where no human bone or less dense bone is present. Therefore, the area of the detector 100 under the denser areas of the bone receives less x-ray flux. A smaller amount of x-ray flux striking the detector 100 causes less x-ray flux to be converted into light by the scintillator 130. The decreased conversion of x-ray flux into light in turn causes less light to strike the matrix of pixels 120, and correspondingly, the photodiodes 125. With less light striking the photodiodes 125, less electrical charge is discharged from the photodiodes 125.
Conversely, in the areas of the detector 100 where there is a smaller density of human bone or no bone, less x-ray flux is blocked out and more x-ray flux strikes the scintillator 130. With a larger x-ray flux striking the scintillator 130, more x-ray flux is converted into light by the scintillator 130. Therefore, more light strikes the matrix of pixels 120, and correspondingly, the photodiodes 125. With more light striking the photodiodes 125, more electrical charge is discharged in the photodiodes 125.
The electrical charge stored in the photodiodes 125 is then read by the readout electronics 140. The readout electronics 140 scan the various pixels in the matrix of pixels 120 to determine the amount of initially stored electrical charge that has been discharged in the photodiodes 125, as described above. The readout electronics 140 convert the amount of discharged electrical charge in the photodiodes 125 into an image signal 170. In general, a large amount of discharged electrical charge in a photodiode 125 results in a high image signal 170 for the corresponding pixel. The image signal 170 is then sent to a data acquisition system 160. Once the data acquisition system 160 receives the image signal 170, the data acquisition system 160 uses the signal 170 to create an electronic x-ray image 180. The electronic x-ray image 180 may then be sent to an output, such as a computer screen.
Generally, photodiodes 125 and correspondingly, pixels, that receive greater intensities of incident light and therefore discharge a greater amount of electrical charge, have a large contrast with neighboring pixels that receive lesser intensities of incident light. The contrast in discharged electrical charge can cause sharp contrast in an x-ray image 180 between areas of the detector 100 where x-rays have struck the detector 100 unimpeded or by passing through objects with a lower density and areas of the detector 100 where x-rays were impeded in their path to the detector 100 by objects with a greater density.
In any imaging system, x-ray or otherwise, image quality is important. Image quality problems may be caused by electrical charge that has become stored, or “trapped,” in areas of the scintillator 130. The trapped charge fills up the trapping centers in the scintillator 130, which can result in an increase of the image signal 170 per exposure, or gain of the scintillator 130. The increase of the gain of the scintillator 130, known as hysteresis, leads to an increase in image signal 170 for constant x-ray exposure.
Hysteresis can result from a large x-ray flux dose, such as 100 mR, to the scintillator 130. In addition, the scintillator 130 material may contribute to hysteresis. For example, scintillators 130 including CsI doped with Tl (“CsI(Tl)”) generally contain deep charge trapping centers. The trapping centers may maintain or “trap” an electrical charge for an extended period of time. The trapped electric charges can cause an increase in the gain in the scintillator 130 and therefore an increase in the image signals 170. As discussed below, an increase in the image signals 170 may result in a shape artifact in the image 180. Gradually, the trapped electrical charge eventually decays enough to avoid significant imaging problems.
However, the deeper a charge-trapping center is, the longer the trapped charge takes to sufficiently decay. For example, the deep charge trapping centers of CsI(Tl) scintillators 130 can cause the decay time for the trapped electrical charges to be a considerably long time. When a significant number of deep charge trapping centers trap a sufficiently large electrical charge, a gain of the scintillator 130 and the image signal 170 can increase for the areas of the scintillator 130 where the deep charge trapping centers are filled with charge. Therefore, a uniform x-ray exposure to the detector 100 may cause increased signal 170 levels in areas with trapped charge. These areas of increased signal 170 levels may appear in images 180 as “ghosts” of previous x-ray exposures and are therefore referred to as “ghost images,” or shape artifacts.
Effects of hysteresis in an x-ray image 180 are exemplified in FIG. 2. FIG. 2 illustrates images 180 from an exemplary detector 100 for 1) a uniform exposure preceding a large non-uniform exposure (image 110), 2) for a large non-uniform exposure (image 120) and 3) for a uniform exposure following the large non-uniform exposure (image 130). For image 110, charge trap centers of the scintillator 130 in the detector 100 have little or no stored charge (that is, a scintillator 130 with normal gain). As discussed above, with little or no trapped charge in the charge trapping centers of the scintillator 130, an image signal 170 may not be affected, and the resultant signal level for the detector 100 appears uniform.
For image 120, the image 180 of a large non-uniform exposure, a top third of the detector 100 is covered with a lead sheet while a bottom two-thirds of the detector 100 is left uncovered. The detector 100 is then exposed to a large x-ray flux dose, for example 100 mR. The lead sheet over the top third of the detector 100 blocks the x-ray flux from striking the detector 100. Accordingly, the x-ray flux is blocked from striking the scintillator 130; and no x-ray flux is converted into light. Therefore, the photodiodes 125 in the top third of the detector 100 do not have any of their initial reverse bias voltage charge discharged by the light. When the readout electronics 140 scan the photodiodes 125, the readout electronics 140 create an image signal 170 that has no x-ray exposure for the photodiodes 125 in the top third of the detector 100. Therefore, when the image signal 170 is converted into the image 120, the top third of the detector 100 appears to be dark, indicating that the top third of the detector 100 was not exposed to an x-ray flux.
Conversely, in the bottom two-thirds of the detector 100 in image 120, a large x-ray flux dose strikes the detector 100, a large dose of x-ray flux strikes the scintillator 130 and a large dose of x-ray flux is converted into light. Therefore, the photodiodes 125 in the bottom two-thirds of the detector 100 discharge a large amount of stored electrical charge. Therefore, the photodiodes 125 in the bottom two-thirds of the detector 100 have a large amount of their initial reverse bias voltage charge discharged by the light. When the readout electronics 140 scan the photodiodes 125 to determine the electrical charge discharged in the photodiodes 125, the readout electronics 140 create an image signal 170 that has a large amount of x-ray exposure for the photodiodes 125 in the bottom two-thirds of the detector 100. Therefore, when the image signals 170 are converted into the image 120, the bottom two-thirds of the detector 100 appear to be bright, indicating that the bottom two-thirds of the detector 100 were exposed to a large amount of x-ray flux.
After waiting an amount of time so that all the electrical charge stored in the photodiodes 125 should be dissipated, the detector 100 may again be exposed to a uniform amount of x-ray flux to produce image 130. The amount of x-ray flux used to produce image 130 can be equivalent to the exposure level used in the first image 110. The readout electronics 140 then scan the photodiodes 125 to detect the amount of electrical charge discharged. Because the x-ray exposure was uniform and equal to that used to obtain the first image, the image signals 170 created by the readout electronics 140 should be uniform across the entire image 130 and display the same signal 170 levels as the image 110 taken prior to the large exposure. That is, as there has been no impedance to the uniform x-ray exposure used to produce either image 110 or image 130, both image 110 and image 130 should appear similar. However, the image signals 170 in image 130 are affected by changes in the scintillator 130 caused by the large nonuniform exposure. That is, as discussed above, a large x-ray flux may cause electrical charges to become trapped in the charge trapping centers of the scintillator 130 in the bottom two thirds of the detector 100; therefore, when the readout electronics 140 scan the detector 100, the bottom two-thirds of the detector 100 may have an increased signal 170. The image signal 170 from the bottom two-thirds of the detector 100 is sent to the data acquisition system 160 and appears in the image 130 as an area of increased signal 170.
FIG. 3 illustrates a graph representing the relationship between an x-ray flux exposure and the increase in gain of a scintillator. The percentage increase in gain of the scintillator 130 represents the increase in gain of exposed portions of the scintillator 130 compared to the gain of unexposed portions of the scintillator 130. As FIG. 3 demonstrates, scintillator 130 hysteresis may cause a significant increase in gain of the scintillator 130. For example, FIG. 3 illustrates that an x-ray exposure may result in an increase in scintillator 130 gain of a few percent.
The shape artifacts or “ghost images” caused by scintillator 130 hysteresis are impediments to the image quality of x-ray images 180. For example, shape artifacts can appear to be non-existent objects in the x-ray image 180. In addition, the artifacts can effectively block or mask the appearance of an object of interest in an x-ray image 180. For example, an artifact may block or mask a tumor in the x-ray image 180 of the human anatomy.
Currently, no known solution exists for correcting scintillator 130 hysteresis. Possible solutions include either altering the material properties of the scintillator 130 or simply waiting for the electrical charge trapped in the charge trapping centers of the scintillator 130 material to decay. Changing the material properties of the scintillator 130 may include changing growth processes of the scintillator 130 material, altering the doping procedures of the dopant in the scintillator 130 and/or changing a microstructure of the scintillator 130. However, each of the proposed solutions is unlikely due to the high cost associated with altering material properties of the scintillator 130. In addition, altering material properties is an uncertain solution for correcting scintillator 130 hysteresis. Altering material properties is not known to definitively prevent or reduce scintillator 130 hysteresis while maintaining high x-ray image 180 quality. Also, as described above, simply “waiting out” the effects of scintillator 130 hysteresis in current systems by waiting until the trapped charges in the charge trapping centers of the scintillator 130 material have sufficiently decayed is an unrealistic solution as some trapped charges may require a considerable amount of time to sufficiently decay.
Therefore, a need currently exists for a method and system for detecting shape artifacts in an x-ray image caused by scintillator hysteresis and for minimizing or eliminating the shape artifacts caused by scintillator hysteresis.